Serna may refuse dropping in a specific position if it is not allowed by the XML Schema constraint.
Where the dropping of the current content in the Editing Window is allowed, you see the Insert Element icon.
Figure 1. Dropping is allowed at the position in Editing Window.
Where the dropping of the current content in the ContentMap is allowed, you see a pointer cursor with the tag of the item.
Where the dropping of the current content in the Editing Window or ContentMap is not allowed , you see a crossed circle.
Figure 2. Dropping is not allowed at the position in Editing Window.
Figure 3. Dropping is not allowed at the position in ContentMap.